The invention relates to an improvement in the handling of tape in a helical video tape recorder when in selected operating modes, and particularly to the prevention of damage to the tape or to the video heads when threading or unthreading tape in the reverse direction about a helical scanner drum.
In helical tape recorders, a problem which occurs all too frequently when threading a tape backwards from the takeup reel to the supply reel of the transport when the scanner drum is stopped, is that the tape tends to slide up and off the scanner drum as it is pulled through the transport during the threading process. This effect is commonly termed "barber pole" in the art, and can cause severe damage to the tape. An additional and similar problem arises in such transports, when tape is withdrawn from the takeup reel and from the tape path in the reverse direction, while the scanner drum is rotating at normal high speed. In such a situation, since the scanner drum rotates in the direction the tape is moving, it pulls the approaching end of the tape from the takeup reel, into the scanner and against the tape guide. The latter condition not only damages the tape but also may destroy the fragile video heads, particularly if the tape end is not smooth and flat. The above condition may occur whenever the scanner is rotating at its normal high speed and the tape is withdrawn in the reverse direction, either during reverse shuttle or reverse playback modes of transport operation.
To date, no attempt has been made to correct the first problem of damage to a tape when performing the reverse threading operation. Operators of tape transports have instead simply tried to exercise special caution when reverse threading tape to prevent the barber pole effect. However as well known the average operator in the field is notoriously careless in handling tape particularly when hurrying. Thus all too often tape is damaged. Attempts have been made in the art to prevent the occurrence of the second problem of damage during the reverse unthreading of tape, but such attempts have not proved very successful. One of the approaches is of a mechanical nature, wherein the tape guides which maintain the tape against the scanner drum are rotated away from the scanner as the end of the tape approaches. This alleviates the above problem associated with unthreading the tape in the reverse direction. However, the retractable guide approach involves the addition of relatively complex mechanical apparatus and associated electrical controls, which combination further tends to be relatively unreliable. Such scheme also fails to prevent the barber pole effect when threading tape in the reverse direction.
In another scheme, the tape transport and thus the tape movement is stopped or slowed to an acceptable low speed as the tape end approaches. In this scheme, the end-of-tape is determined in conventional manner and the drives to the capstan motor and to the reel motors are electrically braked and disabled respectively, to stop the tape while the scanner continues to rotate. Such a scheme generally is possible only on very sophisticated tape transports employing dynamic braking and very powerful reel motors, while further employing relatively complex reel motor servos. This scheme also fails to prevent the barber pole effect when threading tape in the reverse direction.
In another approach, electrical controls are applied both to the reel motors and to the scanner, in response to the determination of a selected nominal reel hub diameter corresponding to end-of-tape. The shuttle speed of the transport is decreased to an acceptable speed, and the scanner drive is turned off to allow the scanner to coast to a stop as the tape is pulled from the scanner. This latter scheme provides moderate but unpredicable protection from the above-mentioned problems, since the scanner is coasting in uncontrolled fashion and thus is rotating at an unknown speed. This is partly due to the fact that the end-of-tape is not precisely known and can vary as much as 100 to 200 wraps about the reel hub. If the scanner rotational speed is excessive at the moment when the tape end is pulled from the scanner, the tape or heads may be damaged. If the rotation is too slow, the tape will barber pole up the scanner also resulting in tape damage. This scheme also fails to prevent the barber pole effect when threading tape in the reverse direction. This latter scheme is illustrated, for example, in the Ampex Corporation, VPR-80 Video Production Recorder Manual 1809548, issued June, 1984.
In transports employing the latter scheme, a scanner tachometer produces a pulse once each revolution, which then causes an associated microprocessor to sample the present scanner period of rotation and angular position with respect to an external reference. These data are used to produce error signals that are fed to the scanner motor to maintain the normal record/playback scanner speed. Because the tachometer pulse occurs only once per revolution, and 16-bit counters are used to measure the period of rotation and angular position, such a scheme cannot measure or control the scanner at speeds below, for example, about 900 revolutions per minute (rpm). Such rotational speed is far too fast to allow safely threading or unthreading the tape in the reverse direction.
Accordingly, the invention combination provides for overcoming the problems discussed above; namely, allows gentle threading and unthreading of the tape while precluding the barber pole effect, when the tape is placed or withdrawn about the scanner drum in the reverse direction. More particularly, a slow speed scanner servo defined by a high resolution scanner tachometer (tach) and an additional counter to measure scanner speed when at substantially reduced values, is employed in combination with the generally conventional scanner servo means mentioned above for supplying the scanner period of rotation and angular position when at normal record/playback scanner speed. The slow speed scanner servo is selectively enabled during the reverse threading and unthreading modes of operation, to servo the scanner to maintain a preselected reduced speed selected from a possible range of speeds substantially below the normal scanner high speed. The pulses from the high resolution scanner tach are counted by the counter, and the counter then is sampled and reset once every television field. This allows measuring scanner speeds down to the order of ten rpm. In addition, the tape speed preferably is selected to also maintain a preselected reduced tape speed, whereby the system maintains a scanner circumferential velocity which is equal to, or slightly greater than, the tape speed. The ratio of speeds may vary commensurate with those preferred to safely perform the reverse threading, or the reverse unthreading, operation.